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Abstract:

Optical systems operable to emit an output beam having fast-switched
wavelengths are provided. In one embodiment, an optical system includes a
laser and a wavelength conversion device. The laser emits a pump beam
that switches between at least two fundamental spectral peaks at
different wavelengths at a wavelength cycling period that is shorter than
a response time of the human eye. The wavelength conversion device
includes a non-linear optical medium configured to phase match the
frequency doubling of the at least two switched fundamental spectral
peaks such that an output beam that switches between at least two
frequency-converted spectral peaks at different converted-wavelengths is
emitted from an output facet of the wavelength conversion device when the
pump beam of the optical source is incident on an input facet of the
wavelength conversion device.

Claims:

1. An optical system comprising a laser and a wavelength conversion
device, wherein: the laser emits a pump beam that switches between at
least two fundamental spectral peaks at different wavelengths at a
wavelength cycling period that is shorter than a response time of the
human eye; and the wavelength conversion device comprises a non-linear
optical medium configured to phase match the frequency doubling of the at
least two switched fundamental spectral peaks such that an output beam
that switches between at least two frequency-converted spectral peaks at
different converted-wavelengths is emitted from an output facet of the
wavelength conversion device when the pump beam is incident on an input
facet of the wavelength conversion device.

2. The optical system of claim 1 wherein the optical system is programmed
to: generate at least a portion of a laser image on a projection surface
by operating the laser for optical emission of encoded image data, the
laser image comprising a plurality of successive image frames temporally
separated by an inter-frame time; and control the laser such that the
pump beam switches between the at least two fundamental spectral peaks
during the inter-frame time.

3. The optical system of claim 2 further comprising an optical scanning
device, wherein the optical system is programmed to generate the scanned
image on the projection surface by controlling the optical scanning
device to scan the output beam across a plurality of image pixels forming
the successive image frames.

4. The optical system of claim 3 wherein the pump beam transitions
between the at least two fundamental spectral peaks at a transition time
that is less than or equal to 4 ms.

5. The optical system of claim 1 wherein: the plurality of successive
frames comprises at least two color-specific image frames that are
successively generated; one of the at least two color-specific image
frames comprises a frequency-converted color image frame; and the
inter-frame time is defined by a time between successive
frequency-converted color image frames.

6. The optical system of claim 1 wherein the laser is operated such that
the at least two fundamental spectral peaks have approximately equal
power.

7. The optical system of claim 1 wherein the at least two
frequency-converted spectral peaks are separated by more than about 0.2
nm in wavelength.

8. The optical system of claim 1 wherein: the laser comprises a
wavelength selective section thermally coupled to a heater device; and
the pump beam switches between the at least two fundamental spectral
peaks by the change of temperature produced by the heating device.

9. The optical system of claim 1 wherein: the laser comprises a
wavelength selective section; and the pump beam switches between the at
least two fundamental spectral peaks by the injection of a modulated
electrical current into the wavelength selective section.

10. The optical system of claim 1 wherein: the wavelength conversion
device is characterized by a first phase matching peak and a second phase
matching peak; the at least two fundamental spectral peaks comprise a
first fundamental spectral peak and a second fundamental spectral peak;
the at least two switched frequency-converted spectral peaks comprise a
first frequency-converted spectral peak and a second frequency-converted
spectral peak; and a response ratio of the first phase matching peak and
the second phase matching peak is such that the first and second
frequency-converted spectral peaks of the output beam have approximately
equal average power when the pump beam is incident on the input facet of
the wavelength conversion device.

11. The optical system of claim 1 wherein: the wavelength conversion
device is characterized by a first phase matching peak, a second phase
matching peak, and a third phase matching peak; the at least two
fundamental spectral peaks comprise a first fundamental spectral peak, a
second fundamental spectral peak, and a third fundamental spectral peak;
the at least two switched frequency-converted spectral peaks comprise a
first frequency-converted spectral peak, a second frequency-converted
spectral peak, and a third frequency-converted spectral peak; and a
response ratio of the first phase matching peak, the second phase
matching peak, and the third phase matching peak is such that the first,
second and third frequency-converted spectral peaks of the output beam
have approximately equal average power when the pump beam is incident on
the input facet of the wavelength conversion device.

12. The optical system of claim 1 wherein: the non-linear optical medium
is quasi-periodically poled and comprises a plurality of poling domains
positioned along a longitudinal optical axis of the wavelength conversion
device in accordance with a phase-modulated periodicity that is
characterized by a phase modulation function superimposed on a carrier
periodicity such that respective positions of at least some of the poling
domains are longitudinally shifted relative to normal periodic positions
defined by the carrier periodicity; the phase-modulated periodicity of
the plurality of poling domains is such that the wavelength conversion
device is characterized by at least two phase matching peaks that produce
the frequency-converted spectral peaks; and at least two of the
frequency-converted spectral peaks have approximately equal power when
the pump beam is incident on the input facet of the wavelength conversion
device.

13. The optical system of claim 12 wherein the phase modulation function
is a rectangular wave phase modulation function with a total modulation
depth of π such that the plurality of domains are phase-modulated by a
periodic sign-reversal of selected domains.

14. The optical system of claim 13 wherein: the wavelength conversion
device is characterized by a first phase matching peak and a second phase
matching peak; and the periodic sign-reversal has a duty cycle of about
0.5.

15. The optical system of claim 12 wherein the phase modulation function
is a rectangular wave phase modulation function with a total modulation
depth other than π.

16. The optical system of claim 15 wherein the total modulation depth of
the rectangular wave phase modulation function is between 0.58.pi. to
about 0.68.pi..

17. The optical system of claim 12 wherein the phase modulation function
is a periodic trapezoidal function.

18. The optical system of claim 17 wherein the periodic trapezoidal
function is characterized by a total modulation depth ranging from about
0.58.pi. to about 1.27.pi..

19. The optical system of claim 17 wherein the periodic trapezoidal
function is characterized by a total modulation depth of about 0.754.pi.
and a plateau duty cycle of about 0.51.

20. The optical system of claim 12 wherein the phase modulation function
is a sinusoidal function such that the plurality of domains is
continuously phase-modulated.

21. The optical system of claim 20 wherein the sinusoidal function is
characterized by a total modulation depth ranging from about 0.80.pi. to
about 1.4.pi..

22. The optical system as claimed in claim 12 wherein the length of the
wavelength conversion device is within a range of about
(m-0.05)*ΛS to about (m+0.6)*ΛS for a symmetric
phase modulation function or within a range of about
(m-0.55)*ΛS to (m+0.05)*ΛS for an anti-symmetric
phase modulation function, where ΛS is the period of the
phase modulation function and m is a non-negative integer.

23. The optical system of claim 1 wherein the spectral bandwidth of the
at least two frequency-converted spectral peaks is greater than or equal
to 0.1 nm.

24. An optical system comprising a laser and a wavelength conversion
device, wherein: the laser emits a pump beam that switches between at
least two fundamental spectral peaks separated by at least 0.4 nm at a
wavelength cycling period; the wavelength conversion device comprises a
non-linear optical medium that is quasi-periodically poled with a
plurality of poling domains positioned along a longitudinal optical axis
of the wavelength conversion device in accordance with a phase-modulated
periodicity that is characterized by a phase modulation function
superimposed on a carrier periodicity such that respective positions of
at least some of the poling domains are longitudinally shifted relative
to normal periodic positions defined by the carrier periodicity; and the
phase-modulated periodicity of the plurality of poling domains is such
that the wavelength conversion device is characterized by at least two
phase matching peaks that phase match the frequency doubling of the at
least two switched fundamental spectral peaks such that an output beam
that switches between at least two frequency-converted spectral peaks
separated by at least 0.2 nm in wavelength is emitted from an output
facet of the wavelength conversion device when the pump beam of the
optical source is incident on an input facet of the wavelength conversion
device; and the optical system is programmed to: generate at least a
portion of a laser image on a projection surface by operating the laser
for optical emission of encoded image data, the laser image comprising a
plurality of successive image frames temporally separated by an
inter-frame time; and control the laser such that the pump beam switches
between the at least two fundamental spectral peaks during the
inter-frame time.

25. An optical system comprising a laser and a wavelength conversion
device, wherein: the laser emits a pump beam that switches between at
least two fundamental spectral peaks at different wavelengths at a
wavelength cycling period, the at least two fundamental spectral peaks
being separated by at least 0.4 nm and having a spectral bandwidth
greater than or equal to 0.2 nm; and the wavelength conversion device
comprises a non-linear optical medium configured to phase match the
frequency doubling of the at least two switched fundamental spectral
peaks such that an output beam that switches between at least two
frequency-converted spectral peaks at different converted-wavelengths
with approximately equal power is emitted from an output facet of the
wavelength conversion device when the pump beam of the optical source is
incident on an input facet of the wavelength conversion device.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. patent application Ser. No.
12/782,205 filed on May 18, 2010, entitled "Multiple Wavelength Optical
Systems," but does not claim priority thereto.

BACKGROUND

[0002] 1. Field

[0003] Embodiments of the present disclosure generally relate to optical
systems such as laser systems. More specifically, the embodiments relate
to optical systems capable of producing an output beam having a switched
wavelength for reducing the appearance of speckle.

[0004] 2. Technical Background

[0005] While blue and red semiconductor lasers are currently readily
available, the progress in nitride semiconductor technology has not yet
resulted in a creation of native green lasers with sufficient output
power, efficiency and cost effectiveness. An attractive alternative is to
use a near-infrared (1060 nm) laser diode and generate green light by
frequency doubling in a nonlinear optical medium such as periodically
poled lithium niobate (PPLN) crystal. This allows for a small package
size and reasonable efficiency, but results in a high level of speckle in
projected images due to the high spectral and spatial coherence of the
laser source.

[0006] Speckle is observed whenever a coherent light source is used to
illuminate a rough surface, for example, a screen, wall, or any other
object that produces a diffused reflection or transmission. Particularly,
a multitude of small areas of the screen or other reflecting objects
scatter light into a multitude of reflected beams with different points
of origination and different propagation directions. At an observation
point, for example in the eyes of an observer or at the sensor of a
camera, these beams interfere constructively to form a bright spot, or
destructively to form a dark spot, producing a random granular intensity
pattern known as speckle. Speckle causes high spatial frequency noise in
the projected image. Speckle may be characterized by grain size and
contrast, the latter usually defined as a ratio of standard deviation to
mean light intensity in the observation plane. For a large enough
illuminated area and a small enough surface roughness, the speckle will
be "fully developed," with a brightness standard deviation of 100%. If an
image is formed on the screen using a coherent light source such as a
laser beam, such granular structure will represent noise resulting in
serious degradation of the image quality. This noise presents a
significant problem, particularly when the projector is used to display
high-spatial-frequency image content, such as text.

[0007] Accordingly, a need exists for optical systems that reduce the
appearance of speckle to improve the image quality of laser projected
images.

SUMMARY

[0008] In one embodiment, an optical system includes a laser and a
wavelength conversion device. The laser emits a pump beam that switches
between at least two fundamental spectral peaks at different wavelengths
at a wavelength cycling period that is shorter than a response time of
the human eye. The wavelength conversion device includes a non-linear
optical medium configured to phase match the frequency doubling of the at
least two switched fundamental spectral peaks such that an output beam
that switches between at least two frequency-converted spectral peaks at
different converted-wavelengths is emitted from an output facet of the
wavelength conversion device when the pump beam of the optical source is
incident on an input facet of the wavelength conversion device.

[0009] In another embodiment, an optical system includes a laser and a
wavelength conversion device. The laser emits a pump beam that switches
between at least two fundamental spectral peaks separated by at least 0.4
nm at a wavelength cycling period. The wavelength conversion device
includes a non-linear optical medium that is quasi-periodically poled
with a plurality of poling domains positioned along a longitudinal
optical axis of the wavelength conversion device in accordance with a
phase-modulated periodicity that is characterized by a phase modulation
function superimposed on a carrier periodicity such that respective
positions of at least some of the poling domains are longitudinally
shifted relative to normal periodic positions defined by the carrier
periodicity. The phase-modulated periodicity of the plurality of poling
domains is such that the wavelength conversion device is characterized by
at least two phase matching peaks that phase match the frequency doubling
of the at least two switched fundamental spectral peaks such that an
output beam that switches between at least two frequency-converted
spectral peaks separated by at least 0.2 nm in wavelength is emitted from
an output facet of the wavelength conversion device when the pump beam of
the optical source is incident on an input facet of the wavelength
conversion device. The optical system is programmed to generate at least
a portion of a laser image on a projection surface by operating the laser
for optical emission of encoded image data, the laser image comprising a
plurality of successive image frames temporally separated by an
inter-frame time, and to control the laser such that the pump beam
switches between the at least two fundamental spectral peaks during the
inter-frame time.

[0010] According to yet another embodiment, an optical system includes a
laser and a wavelength conversion device. The laser emits a pump beam
that switches between at least two fundamental spectral peaks at
different wavelengths at a wavelength-switching time, the at least two
fundamental spectral peaks being separated by at least 0.2 nm at a
wavelength-switching rate and having a spectral bandwidth greater than or
equal to 0.2 nm The wavelength conversion device includes a non-linear
optical medium configured to phase match the frequency doubling of the at
least two switched fundamental spectral peaks such that an output beam
that switches between at least two frequency-converted spectral peaks at
different converted-wavelengths with approximately equal power is emitted
from an output facet of the wavelength conversion device when the pump
beam of the optical source is incident on an input facet of the
wavelength conversion device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic diagram of an optical system according to one
or more embodiments shown and described herein;

[0012]FIG. 2 is a graph depicting a combined optical spectrum of the
frequency converted output beam produced by one or more embodiments as
shown and described herein;

[0013]FIG. 3A is a graph depicting a square wave signal applied to a DBR
heater of a DBR laser according to one or more embodiments shown and
described herein;

[0014]FIG. 3B is a graph depicting the optical output waveforms of a DBR
laser driven by the square wave signal depicted in FIG. 3A;

[0015]FIG. 4A is a graph depicting spectral separation between two
switching wavelengths versus amplitude of a square wave applied to a DBR
heater according to one or more embodiments shown and described herein;

[0016]FIG. 4B is a graph depicting switching speed of a DBR laser versus
wavelength separation according to one or more embodiments shown and
described herein;

[0017] FIGS. 5 and 6 are graphs depicting an optical spectrum of a pump
beam having two fundamental spectral peaks according to one or more
embodiments shown and described herein;

[0018] FIG. 7 is a schematic diagram of a wavelength conversion device
having a phase-modulated periodicity defined in part by a discrete phase
modulation function according to one or more embodiments shown and
described herein;

[0019]FIG. 8A is a graph depicting a discrete phase modulation function
according to one or more embodiments shown and described herein;

[0020]FIG. 8B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the discrete phase modulation function
illustrated in FIG. 8A according to one or more embodiments shown and
described herein;

[0021]FIG. 8c is a graph depicting a wavelength spectral response of a
wavelength conversion device having a phase-modulated periodicity defined
in part by the rectangular phase modulation function illustrated in FIG.
8A according to one or more embodiments shown and described herein;

[0022]FIG. 9A is a graph depicting a discrete phase modulation function
having a duty cycle near 0.3 according to one or more embodiments shown
and described herein;

[0023]FIG. 9B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the discrete phase modulation function
illustrated in FIG. 9A according to one or more embodiments shown and
described herein;

[0024]FIG. 10A is a graph depicting a rectangular symmetric phase
modulating function according to one or more embodiments shown and
described herein;

[0025]FIG. 10B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the discrete phase modulation function
illustrated in FIG. 10A according to one or more embodiments shown and
described herein;

[0026] FIG. 11A is a graph depicting a trapezoidal phase modulation
function according to one or more embodiments shown and described herein;

[0027]FIG. 11B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the discrete phase modulation function
illustrated in FIG. 11A according to one or more embodiments shown and
described herein;

[0028]FIG. 12A is a graph depicting a continuous sinusoidal
phase-modulating function according to one or more embodiments shown and
described herein;

[0029]FIG. 12B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the discrete phase modulation function
illustrated in FIG. 12A according to one or more embodiments shown and
described herein;

[0030]FIG. 13 is a schematic diagram depicting a wavelength conversion
device having a phase-modulated periodicity defined in part by a phase
modulating function according to one or more embodiments shown and
described herein;

[0031]FIG. 14A is a graph depicting a continuous sinusoidal
phase-modulating function according to one or more embodiments shown and
described herein;

[0032]FIG. 14B is a graph depicting a spectral response in wave-vector
space of a wavelength conversion device having a phase-modulated
periodicity defined in part by the sinusoidal function illustrated in
FIG. 14A according to one or more embodiments shown and described herein;
and

[0033]FIG. 14c is a graph depicting a measured wavelength spectral
response of a wavelength conversion device designed to have a
phase-modulated periodicity defined in part by the sinusoidal function
illustrated in FIG. 14A according to one or more embodiments shown and
described herein;

[0035] FIGS. 16A-16C are graphs depicting output optical spectra of an
optical system having a pump beam that switches between three wavelengths
according to one or more embodiments shown and described herein;

[0036]FIG. 17 is a graph depicting a measured tuning curve of a
quasi-phase matching wavelength conversion device according to one or
more embodiments described and illustrated herein;

[0037]FIG. 18A is a graph depicting a measured image histogram of an
optical system pumped by a DBR laser with a switched-wavelength output
according to one or more embodiments described and illustrated herein;

[0038] FIG. 18 B is a graph depicting measure image histogram of an
optical system pumped by a DBR laser operated in CW mode; and

[0039]FIG. 19 is a graph depicting second harmonic generation conversion
efficiency at high peak power of uniform period poling structure and a
quasi-phasematching structure according to one or more embodiments
described and illustrated herein.

DETAILED DESCRIPTION

[0040] Embodiments described herein generally relate to optical systems
that may reduce the appearance of speckle in images when incorporated
into a laser projector system. Although embodiments described herein may
be described in the context of laser projection systems, embodiments are
not limited thereto; embodiments described herein may be incorporated
into systems other then laser projector systems.

[0041] Reference will now be made in detail to embodiments of the present
disclosure, examples of which are illustrated in the accompanying
drawings. Whenever possible, the same reference numerals will be used
throughout the drawings to refer to the same or like parts. The optical
system generally comprises at least one semiconductor laser, optional
coupling optics, and a wavelength conversion device. A package controller
may be included to operate the semiconductor laser and/or coupling
optics. The output of the semiconductor laser is optically coupled into
the input of the wavelength conversion device either directly or by the
use of the coupling optics. The semiconductor laser produces a pump beam
that switches between at least two fundamental spectral peaks separated
by ΔλF. The wavelength conversion device converts the
energy of the pump beam emitted by the semiconductor laser into an output
beam having at least two frequency-converted spectral peaks. Various
components and configurations of the optical system will be further
described herein.

[0042]FIG. 1 generally depicts an optical system 100 described herein. A
pump beam 120 emitted by the semiconductor laser 110 may have two or more
switched fundamental spectral peaks in the infrared wavelength band. The
pump beam 120 may be either directly coupled into a waveguide portion 137
of the wavelength conversion device 130 or can be coupled into the
waveguide portion of wavelength conversion device 130 using adaptive
optics, illustrated as first and second coupling optics 121 (e.g., lenses
122a and 122b). The wavelength conversion device 130 converts the output
wavelengths of the pump beam 120 into higher harmonic waves and produces
a visible output beam 140 having at least two switched
frequency-converted spectral peaks 142 and 146 (FIG. 2). This type of
optical package is particularly useful in generating shorter wavelength
laser beams from longer wavelength semiconductor lasers and can be used,
for example, as a visible light source for laser projection systems.

[0043] The optical source 110 may comprise one or more lasers, such as
distributed feedback (DFB) lasers, distributed Bragg reflector (DBR)
lasers, vertical cavity surface-emitting lasers (VCSEL), vertical
external cavity surface-emitting lasers (VECSEL) or Fabry-Perot lasers,
for example. In addition, if the laser gain medium is a semiconductor
medium, it may involve the use of carrier confinement in quantum wells,
quantum wires, or quantum dots. In laser projection system applications,
the optical source may comprise three semiconductor lasers: a first
semiconductor laser to emit a beam in the red spectral range, a second
semiconductor laser to emit a beam in the blue spectral range, and a
third semiconductor laser to emit a beam in the infrared spectral range,
which is then frequency up-converted into one or more frequency-converted
spectral peaks in a frequency up-converted range (e.g., the green or
yellow spectral range). The optical source 110 and optical system 100 may
be programmed and operated together with scanning or image forming optics
(not shown in figures) to generate a laser projected image comprising a
plurality of pixels across a projection surface.

[0044] In some instances, speckle may be present in a laser-produced
image. Speckle results from random interference--light reflected by
random roughness features on the screen surface can interfere
constructively and destructively, causing bright and dark sports to
appear in the image. If light of different wavelengths is present in the
beam illuminating the screen, the interference can be constructive for
one wavelength and destructive for another one, partially or completely
canceling the net effect. Combining N uncorrelated speckle patterns in
the image can help reduce the speckle contrast by N. The two speckle
patterns produced by two wavelengths separated by Δλ are
correlated by less than 1/e2 if:

Δλ ≧ 1 2 2 π λ _
2 σ h , Eq . ( 1 ) , ##EQU00001##

where σh is the standard deviation of the screen surface local
height (measure of roughness). Assuming mean wavelength of about 530 nm
and roughness at the projection surface of about 100 μm, the
wavelength separation of the frequency-converted spectral peaks should be
greater than or equal to about 0.56 nm. If three wavelengths are present
in the laser beam, and the separation between them satisfies Eq. (1),
then the expected speckle contrast may be reduced approximately by a
factor of {square root over (3)}. Therefore, the wavelength separation
should be large enough to achieve the desired speckle contrast reduction.
As noted below, on some screen surfaces, the wavelength separation
between two visible-light speckle patterns needs to be only about
0.35-0.4 nm to make the patterns uncorrelated (independent) for speckle
reduction purposes. In addition, theoretical calculations predict that
with smaller wavelength separation such as 0.2-0.3 nm, which leads to
partially correlated speckle patterns, some smaller but appreciable
amount of speckle-contrast reduction may be achieved.

[0045] For human eyes, perceived speckle patterns are averaged effect over
a certain time period (i.e., the response time of the human eye). If a
laser can flip-flop its wavelength between two lines within this period,
and the spectral separation between two lines satisfies Eq. (1), then a
perceived speckle contrast reduction of 2 can be achieved. If a laser
can switch its wavelength between three lines within this period, and the
spectral separation between each pair of adjacent lines satisfies Eq.
(1), a perceived speckle contrast reduction of {square root over (3)} is
expected. The response time of the human eye is on the order of about 30
ms to about 70 ms. A wavelength cycling period is the period in which all
of the wavelengths outputted by the laser are cycled. The wavelength
cycling period should be less than the response time of the human eye, or
less than about 30 ms to about 70 ms.

[0046] Therefore, in a laser-beam scanning projector, if an optical system
can switch its wavelength between two lines with a separation meeting Eq.
(1) such that one frame is imaged by one wavelength and the next frame is
imaged by another wavelength, a speckle contrast reduction of {square
root over (2)} is achieved. If an optical system switches its wavelength
between three lines with an adjacent spectral peak separation meeting Eq.
(1) for consecutive frames, then the expected speckle contrast reduction
is by a factor of {square root over (3)}.

[0047] The laser should switch from one wavelength to another wavelength
within the "blank" interval between two successive image frames (i.e.,
the inter-frame time). For example, in a laser-beam scanning projector
system with a 60 Hz frame rate, the typical inter-frame time from the end
of one frame to the beginning of the next frame is about 4 ms. Therefore,
the pump wavelength of the laser should switch from one wavelength to
another wavelength over a transition time that is less than 4 ms. It is
noted that the optical system 100 may be incorporated into projector
systems other than laser-beam scanning projectors, such as frame
projectors systems (e.g., DLP projectors). As such, the inter-frame time
may be longer than that of a laser-beam scanning projector. For example,
a frame projector may project a plurality of successive image frames with
each image frame having a different color (e.g., a red frame, a blue
frame, and a green frame). The transition time of the laser 110 should be
shorter than the inter-frame period between successive green frames
(i.e., the color of the frame that is produced by a frequency-converted
laser beam). In some embodiments, the transition time should be less than
30 ms.

[0048] Referring once again to FIG. 1, the optical source 110 is
configured as a semiconductor laser that can be controlled to fast-switch
the wavelength of its pump beam 120 between at least two fundamental
spectral peaks separated by at least 0.2 nm, preferably 1 nm or more. The
transition time between the two wavelengths should be less than the
interval between two successive image frames (typically 4 ms). In the
illustrated embodiment, a three-section 1060 nm DBR semiconductor laser
110 is used to pump a MgO-doped lithium niobate wavelength conversion
device 130. The DBR laser 110 illustrated schematically in FIG. 1
comprises a wavelength selective section 112 (i.e., DBR section), a phase
section 114, and a gain section 116. The wavelength selective section
112, which can also be referred to as the DBR section of the laser 110,
typically comprises a first order or second order Bragg grating
positioned outside the active region of the laser cavity. This section
provides wavelength selection, as the grating acts as a mirror whose
reflection coefficient depends on the wavelength. The gain section 116 of
the DBR laser 110 provides the major optical gain of the laser and the
phase section 114 creates an adjustable phase shift between the gain
material of the gain section 116 and the reflective material of the
wavelength selective section 112. The phase section 114 may also be used
to reduce the thermal coupling between the gain section 116 and the DBR
section 112 to control the pump-beam wavelength. The wavelength selective
section 112 may be provided in a number of suitable alternative
configurations that may or may not employ a Bragg grating.

[0049] Respective control electrodes 102, 104, 106, may be incorporated in
the wavelength selective section 112, the phase section 114, the gain
section 116, or combinations thereof, and are merely illustrated
schematically in FIG. 1. It is contemplated that the electrodes 102, 104,
106 may take a variety of forms. For example, the control electrodes 102,
104, 106 are illustrated in FIG. 1 as respective electrode pairs but it
is contemplated that single electrode elements 102, 104, 106 in one or
more of the sections 112, 114, 116 will also be suitable for practicing
particular embodiments of the present invention. The control electrodes
102, 104, 106 can be used to inject electrical current into the
corresponding sections 112, 114, 116 of the laser 110. The injected
current can be used to alter the operating properties of the laser by,
for example, controlling the temperature of one or more of the laser
sections, injecting electrical current into a conductively doped
semiconductor region defined in the laser substrate, controlling the
index of refraction of the wavelength selective and phase sections 112,
114 of the laser 110, controlling optical gain in the gain section 116 of
the laser, etc.

[0050] In one embodiment, the DBR section control electrode 102 comprises
a resistive heater that is configured to thermally change the refractive
index and possibly the pitch of the grating of the DBR section 112. To
obtain the fast switching of the wavelength of the DBR laser 110, an
electrical square-wave signal is applied to the DBR resistive heater
electrode to switch the output pump beam 120 of the laser 110 between two
or more wavelengths. FIG. 3A depicts an exemplary square-wave signal
applied to the resistive heater, while FIG. 3B depicts the waveforms of
the laser 110 output pump beam 120 having a first wavelength
λ1 and a second wavelength λ2. As shown in FIG. 3B,
the pump beam 120 alternates between the first wavelength λ1
and the second wavelength λ2. The vertical axis of FIG. 3B is
the intensity of the pump beam 120 in arbitrary units (A.U.). The
frequency of the signal may be equal to the frame rate. For example, in
one embodiment, the frequency of a square wave signal may be chosen at 65
Hz, which is close to the typical frame rate of a laser-beam scanning
projector.

[0051] It should be understood that signals other than a square-wave
signal may be applied to the resistive heater depending on the desired
output of the laser 110 and the wavelength conversion device 130. For
example, the optimum heater electrical signal may not be truly a square
wave when a near-square-wave frequency-doubled output is desired. To
achieve nearly square-wave wavelength changes in the output of the laser
110 with minimal transition time, the signal applied to the DBR section
control electrode 102 may deviate from an ideal square-wave during the
transition time. However, for the remainder of the switching period the
electrical signal may be approximately constant. As an example and not a
limitation, the rising edge of the heater electrical signal may reach a
higher amplitude than the average amplitude of the waveform and then
taper down to a flat plateau similar to that of a square-wave signal.
Further, when the wavelength of the laser is controlled by heat, the
wavelength change is proportional to heater power. When the operating
point of the laser is changed (e.g., ambient temperature change,
operational temperature change), the voltage bias and voltage amplitude
of the signal may need to be changed to compensate. For example, when the
laser is operated at different power levels, the amount of heat
transferred from the gain section to the DBR section varies so that the
heater control has to compensate to maintain the desired wavelength.
Additionally, the heater resistance and/or efficiency may also change or
time and temperature. To achieve an equivalent of power control rather
than voltage or current, in some embodiments a square root operation may
be applied to a computed control signal before it is applied to the
driver circuit.

[0052] The spectral separation of the two wavelengths can be changed by
adjusting the amplitude of the electrical square-wave signal. FIG. 4A
illustrates the spectral separation between two switching wavelengths
versus the amplitude of the square wave signal applied to the resistive
heater of the DBR section 112. FIG. 4B illustrates the switching time
versus the wavelength separation between the different wavelengths. It
may be seen that the switching time between two wavelengths with 1 and 2
nm separations are approximately 1.4 ms and 2.0 ms, respectively. Both
switching times are shorter than the typical 4-ms interval between two
frames in a laser-beam scanning projector. Therefore, the wavelength of
the laser 110 may be switched by applying a modulation signal (e.g., a
square-wave signal) to a heater associated with the DBR section 112 to
thermally change the refractive index and possibly the pitch of the
grating.

[0053]FIG. 5 depicts an output optical spectrum of a wavelength-switched
three-section DBR laser comprising a first fundamental spectral peak 123a
at a first wavelength λ1=1059.04 nm and a second fundamental
spectral peak 125a at a second wavelength λ2=1060.17 nm
separated from λ1 by 1.1 nm (8 measurements averaged). As an
example and referring once again to FIGS. 3A and 3B, it may be seen that
the switching time between two wavelengths is about 1.5 ms. An output
optical spectrum of the laser with its wavelength switching between a
first fundamental spectral peak 123b and a second fundamental spectral
peak 123b with 2-nm spectral separation is shown in FIG. 6.

[0054] In another embodiment, the wavelength of the three-section DBR
laser may be switched by injecting a modulated electrical current into
the DBR section 112. Because of carrier-induced refractive-index-change
effect, the wavelength reflected by the DBR grating can be modulated by
the injection current. The shift (ΔλB) of the Bragg
wavelength (λB) can be expressed in terms of the shift
(Δnd) of the refractive index of the DBR section (nd) as:

Δλ B = Δ n d n d λ B
, Eq . ( 2 ) . ##EQU00002##

[0055] The change of the refractive index (Δnd) with the
carrier density of the DBR section Nd is given by:

Δ n d = Γ n d N d N d
, Eq . ( 3 ) , ##EQU00003##

[0056] where Γ is the optical confinement factor and
dnd/dNd is the material coefficient which describes the
free-carrier plasma effect. The carrier density related to the injection
current (I) may be expressed as:

where Vd, τe, B, and C are the waveguide volume of the DBR
section, carrier lifetime, radiative, and Auger recombination
coefficients in the passive waveguide, respectively. The carrier lifetime
in the passive waveguide is typically on the order of 10 ns. The
switching time between wavelengths with a few nanometer spacing is hence
on the order of about 10 ns. This switching time is much shorter than
typical 4 ms interval between two frames. Therefore, to reduce the
appearance of speckle, the wavelength of the laser 110 may also be
switched by applying current injection into the DBR section 112.

[0057] Referring once again to FIG. 1, the wavelength-conversion device
130 may be configured as second harmonic generation (SHG) crystal with
multiple phase matching peaks that match the wavelengths of the
fundamental spectral peaks of the pump beam 120. The
wavelength-conversion device 130 may comprise a birefringent crystal to
provide birefringent phase matching, or a non-linear optical medium that
is poled at a quasi-phase-matching (QPM) periodicity to quasi-phase-match
the frequency of up-converted light produced at different locations along
the propagation direction. Although embodiments are described hereinbelow
in the context of QPM, it should be understood that other phase-matching
techniques may be utilized (e.g., birefringent phase matching by the use
of birefringent crystals, or inter-modal phasematching based on balancing
the inter-modal dispersion in a waveguide against the material
dispersion).

[0058] In one embodiment, the wavelength conversion device 130 comprises
an SHG crystal with multiple sections in which each section is uniformly
poled to quasi-phase match one of the fundamental spectral peaks. In
another embodiment, the wavelength conversion device 130 comprises a
non-uniformly (i.e., aperiodically) poled SHG crystal according to a
proper design to simultaneously QPM all the wavelengths of all of the
fundamental spectral peaks produced by the laser 110. Both embodiments
may be mathematically described using a model of a nonlinear medium in
which the phase-matching or quasi-phase matching is manipulated by
phase-modulating the nonlinear polarization along the
wavelength-conversion device according to a phase-modulating function
(PMF) which may be continuous or discrete. Phase-modulation of the
nonlinear polarization results in phase modulation of the nonlinear
optical response.

[0059] As shown in FIG. 1, the pump beam may be focused and directed
toward the wavelength-conversion device by coupling optics 121. In the
embodiment shown in FIG. 1, the coupling optics generally comprises a
first lens 122a that collimates the pump beam 120 emitted by the
semiconductor laser 110 and a second lens 122b that focuses the pump beam
120 into the waveguide portion 137 of the wavelength conversion device
130. However, it should be understood that other coupling methods and
devices may be used. Further, the wavelength-conversion device 130 may
comprise a bulk nonlinear optical material, and it may be incorporated
into the laser as an intra-cavity wavelength conversion device. The
lenses may be coupled to an actuator (not shown) for adjusting the
position of the lenses in the x- and y-directions such that the positions
of the lenses are adjustable. Adjusting the position of the lenses in the
x- and y-directions may facilitate positioning the pump beam 130 along
the input facet 131 of the wavelength conversion device and, more
specifically, on the waveguide portion 137 of the wavelength conversion
device 130, such that the pump beam is aligned with the waveguide portion
137 and the frequency-converted output beam 140 of the wavelength
conversion device 130 is optimized. Although the optical system
illustrated in FIG. 1 has a substantially linear orientation, other
orientations and configurations are also possible. For example, the
semiconductor laser 110 and wavelength conversion device 130 may be
oriented such that the optical path of the pump beam 120 and
frequency-converted output beam 140 is a folded optical path.

[0060] The wavelength conversion device 130 may include a nonlinear
optical medium with phase-modulated nonlinear response, such that the
spectrum of its nonlinear optical response contains multiple
phase-matching peaks, properly spaced in wavelength to provide
phase-matching for SHG of green wavelengths spaced adequately for speckle
reduction. The phase-modulation of the nonlinear response may be obtained
by modulating the nonlinear, linear, or both optical properties of the
nonlinear optical medium used for optical-frequency mixing.

[0061] A waveguide portion 137 of the wavelength conversion device 130
extends from the input facet 131 to an output facet 138. The wavelength
conversion device 130 may comprise a crystal formed of a nonlinear
optical material having a plurality of domains with alternating sign of
the nonlinear optical response. Nonlinear optical materials suitable for
a wavelength conversion device 130 may include, but are not limited to,
poled doped or non-doped lithium niobate, poled doped or non-doped
lithium tantalate, and poled doped or non-doped potassium titanyl
phosphate (KTP), for example.

[0062] The light propagation in the wavelength conversion device 130 may
be bulk or optical waveguide propagation. The wavelength conversion
device 130 may comprise a crystal utilizing a method of phase-matching,
including, but not limited to, birefringent, inter-modal, or
quasi-phase-matching. The role of phase-matching is to produce
constructive interference of the electromagnetic waves at the
frequency-converted optical frequency generated by the nonlinear
polarization produced by the fundamental optical field along the optical
path. The phase-modulation of the nonlinear response serves to distribute
the phase-matching into several phase-matching peaks, corresponding to
different optical frequencies, such that each of the phase-matching peaks
provides partial phase-matching. The term partial here means that for a
particular optical frequency corresponding to such a partial
phase-matching peak, the frequency of up-converted optical waves produced
by some, but not necessarily all regions along the crystal, interfere
constructively to produce substantially non-zero up-converted signal on
the output. The phase-modulation thus reduces the maximum phase-matching
at the optical frequency of the frequency-conversion process
phase-matched in the basic, un-modulated design to partial phase-matching
or no phase-matching, while at the same time allowing partial
phase-matching at other optical frequencies for which no phase-matching
was provided by the un-modulated design.

[0063] In the low-conversion limit, the spectrum of the phase-modulated
phase-matching response to a tuned monochromatic input signal is
proportional to the Fourier-transform of the phase-modulating function
(PMF), irrespective of the physical mechanism used for phase matching. At
high-level of energy conversion to the up-converted frequency range, the
spectrum of the up-converted signal deviates from the Fourier-transform
of the PMF. In many cases, this deviation may lead to a minor
deterioration in the speckle contrast reduction. If this deterioration
becomes important, the magnitude ratio of the phase-matching peaks may be
adjusted by changing the PMF such that the spectral response at high
conversion leads to better reduction of speckle contrast, while the
speckle contrast at lower conversion may be somewhat higher, e.g., when
maximum speckle reduction may be more important at higher optical power.

[0064] The PMF imparts spatially varying phase on the frequency
up-converted optical field. This is achieved by modulating the phase or
spatial location of the nonlinear polarization (for example by modulating
the poling of quasi-phase matching crystals) and/or by modulating the
phase delay of up-converted waves generated at different locations along
the crystal by modulating the (effective) refractive index of the medium
or optical path length that they traverse. Some of the techniques for
phase modulation, such as longitudinal shifting of the positions of
inverted domains of quasi-periodic poling, may be interpreted as
embodiments of the nonlinear or linear path of modulation.

[0065] Generally, the nonlinear medium is designed to allow phase-matching
for second harmonic generation (SHG) of two or more optical frequencies,
e.g., 2ω1, 2ω2, 2ω3, etc. Therefore, the
time-averaged frequency-converted output may comprise two or more
frequency-converted spectral peaks having frequencies of 2ω1,
2ω2, 2ω3, and so on.

[0066] Referring to FIG. 7, quasi-phase matching may be achieved by
introducing periodic or quasi-periodic sign reversal of the nonlinear
optical response, for example, by quasi-periodically inverted
ferroelectric domains 132a, 132b within the nonlinear optical material
(e.g., within the waveguide region of the crystal) of the wavelength
conversion device 130. The quasi-periodic poling provides the
quasi-periodic inverting of the sign of the nonlinear coefficient of the
wavelength conversion device in order to ensure constructive addition of
the nonlinear optical response at the frequencies of interest generated
along the device length.

[0067] As illustrated in FIG. 7, the domains may have either positive
(e.g., 132a) or negative (132b) nonlinear polarization associated
therewith. The sign of the nonlinear response of the domains may
alternate approximately periodically along a longitudinal length of the
crystal. As described in more detail below, the periodicity of the
plurality of poling domains may be phase-modulated such that the domains
are quasi-periodically poled. It is noted that the size of the
quasi-periodic domains is exaggerated in FIG. 7 for illustrative
purposes. Further, the wavelength conversion device is only partially
illustrated in FIG. 7.

[0068] Wavelength conversion devices utilized for second harmonic
generation are poled at a phase-matching periodicity to phase-match the
frequency up-converted light produced at different locations along the
propagation direction. To illustrate, the spectral intensity response of
a QPM structure with periodically inverted ferroelectric domains in
lithium niobate with a fixed phase-matching period Λ has QPM peaks
in wave-vector space at m2πΛ where m=1, 2, 3, . . . . The QPM
peak characterized by m=1 has the highest magnitude of each of the QPM
peaks. The relative magnitude of intensity of the QPM peaks corresponding
to different orders m decreases as 1/m2. Therefore, for most
efficient QPM, the QPM peak with m=1 should be matched to the infrared
wavelength of the pump beam emitted by the semiconductor laser to produce
a frequency-converted output beam. It should be understood that the
phase-matching concepts described below may also be applied to
phase-matching techniques other than quasi-phase-matching (e.g.,
birefringent or inter-modal phase matching).

[0069] For illustration purposes, QPM may be described in wave-vector
(k-vector) space. The source of frequency-converted radiation is the
nonlinear dielectric polarization at the up-converted (doubled)
frequency. At a particular moment in time, the phase distribution of this
source wave at an example frequency 2ω along the propagation
direction can be described with a wave-vector
2k.sub.ω=2ωn.sub.ω/c=4πn.sub.ω/λ.sub..om-
ega., where c is the speed of light, λ.sub.ω is the wavelength
in vacuum of an optical wave with frequency ω, and n.sub.ω is
the refractive index of the nonlinear optical medium at the optical
frequency ω of the pump wave producing the nonlinear polarization.
In the case of waveguide propagation and interaction, n.sub.ω is
the effective index of the waveguide mode used to describe the
propagation of the fundamental-frequency (pump) wave causing the
nonlinear polarization. At the same time, the free-propagating
frequency-converted light at frequency 2ω generated at any location
along the propagation direction can be described by a plane wave with
wave-vector
k2ω=2ωn2ω/c=2πn2ω/λ2-
ω=4πn2ω/λ.sub.ω, where n2ω is
the (effective) refractive index of the medium at the second harmonic
frequency 2ω, and λ2ω=λ.sub.ω/2 is the
wavelength in a vacuum of the second harmonic. It should be understood
that free-propagating frequency up-converted light also includes the case
of waveguide propagation, where the optical wave is confined in the
transverse dimensions. If the wave-vector of the source wave is the same
as the wave-vector of the free-propagating waves, then constructing
interference of the generated second harmonic waves is observed along the
device length, and the second harmonic power grows. Otherwise, the second
harmonic power oscillates along the length, reaching only a small maximum
value dictated by the wave-vector mismatch

Δk=k2ω-2k.sub.ω, Eq. (5).

[0070] In cases where Δk is non-zero, one way to allow the second
harmonic power to grow is to use quasi-phase matching as described above.
A periodic reversal of the sign of the nonlinear coefficient with period
Λ and associated k-vector Kg=2π/Λ leads to a
periodic compensation of the phase mis-match caused by Δk.
Quasi-phase matching is achieved when:

k2ω-2k.sub.ω±mKg=0, Eq. (6),

whereby the wave-vector mismatch is compensated. Here m can be any
integer and signifies the quasi-phase matching order.

[0071] As defined by Eq. 5, associated with each fundamental optical
wavelength λ.sub.ω is a wave-vector mismatch Δk for
frequency doubling of the related optical field. In the case of type-I
quasi-phase matching for utilizing the d33 nonlinear coefficient of
lithium niobate, when the fundamental wavelength is on the order of 1060
nm, Δk is significant, on the order of 9000 cm-1. In other
cases of phase-matching, such as birefringent phase-matching, Δk is
0. When describing QPM device design in terms of a carrier periodicity
and its phase modulation, it may be convenient to define in k-vector
space a deviation k-vector by the equation:

δk=Δk±Kc, Eq. (7),

where Kc is the wave-vector describing the carrier periodicity of
QPM. The plus/minus sign in the right-hand-side of Eq. 7 is chosen such
that δk=0 when the phase-matching is not modulated. Since for a
particular design of the nonlinear device the detuning from the central
(design) optical frequency determines the associated phase mismatch, the
spectral response of a nonlinear optical device may be designed and
plotted as a function of δk. In addition, description of the
spectral response due to a particular PMF in terms of δk is
generally valid for all types of phasematching, not only for QPM.

[0072] The mapping between δk and the fundamental wavelength may be
given by the relations:

where ng.sub.ω and ng2ω stand for the group
index at the fundamental and at the second harmonic frequencies. Group
index at frequency ω or wavelength λ may be defined as:

n g = n + ω n ω = n - λ
n λ , Eq . ( 9 ) . ##EQU00006##

Typical values of dΔk/dλ for reverse-proton exchanged
waveguides in MgO-doped lithium niobate are about 28.6 cm-1/nm. For
ridge waveguides confined in an MgO-doped slab sandwiched between layers
with low refractive index, such as silicon dioxide, typical values of
dΔk/dλ are about 26.4 cm-1/nm.

[0073] A truly periodically poled device of length L will have a spectral
response curve in terms of δk described with a sinc2 function
with a full-width at half-maximum (FWHM) equal to 1.772π/L. In terms
of fundamental wavelength, the FWHM is then:

Δλ FWHM = 0.443 n 2 ω g - n ω g
λ 2 L , Eq . ( 10 ) . ##EQU00007##

[0074] Utilizing the above, wavelength conversion devices of the present
disclosure convert the frequency of a pump beam hopping between at least
two fundamental spectral peaks into an output beam similarly hopping
between at least two frequency-converted (frequency-doubled) spectral
peaks. Therefore, the wavelength conversion device should provide two or
more phase matching peaks with approximately equal peak conversion
efficiency (since it is desired that the output power does not change
when the wavelength is changed). Focusing on one of the phase-matching
spectral peaks described above with respect to second harmonic generation
by quasi-phase matching (using QPM order m=1 for example), its shape can
be changed from a single peak to a split peak with multiple spectral
components by altering the character of the QPM crystal domain structure
from strictly periodic at the phase-matching periodicity Λ to a
quasi-periodic structure. Several techniques for manipulating the shape
of the QPM peak may be utilized, including, but not limited to, QPM
gratings with frequency chirp, periodic or aperiodic superlattice,
quasi-periodic superlattice, non-periodic superlattice, and phase
modulation. Additionally, techniques may utilize computer optimization to
obtain a QPM structure with a desired spectral response by utilizing the
Fourier-transform relation between the spectral response and the
distribution of nonlinear susceptibility in the physical space.

[0075] Referring generally to FIGS. 8A-8C, a discrete sign-flip PMF for a
two-peak QPM spectral response with optimized SHG efficiency and the
corresponding spectral response of a wavelength conversion device in
terms of δk and wavelength according to one embodiment are
illustrated. The spectral response is characterized by at least two phase
matching peaks (e.g., quasi-phase matching peaks): a first quasi-phase
matching peak 162 and a second quasi-phase matching peak 164. The first
quasi-phase matching peak 162 corresponds to a first frequency-converted
spectral peak having a frequency of 2ω1 produced by second
harmonic generation, and the second quasi-phase matching peak 164
corresponds to a second frequency-converted spectral peak having a
frequency of 2ω2 produced by second harmonic generation. The
magnitudes of these quasi-phase matching peaks are approximately equal.
It should be noted that the value of the PMF outside the QPM region is
zero since no phase is imparted. The effective nonlinear response outside
the QPM region is also zero. It should be understood that in all plots
where the PMF is shown to equal zero outside a central region of the plot
occupied by the QPM region, the effective nonlinear response is also zero
outside of that QPM region (see FIGS. 8A, 9A, 10A, 11A, 12A, and 14A).

[0076] Modulation of the phase-matching periodicity may be utilized to
obtain a wavelength conversion device having two or more spectral peaks
having approximately equal SHG response to produce frequency-converted
spectral peaks having substantially equal average power. Average power is
used herein as the average optical power of the beam on a projection
surface as viewed by an observer. The wave-vector mismatch Δk
between the infrared (fundamental) and converted (second harmonic) light
is several thousand cm-1. To compensate for this mismatch, a
periodic poling with a short quasi-phase matching period Λ and a
wave-vector Kc of several thousand cm-1 matching Δk may
be required to eliminate the phase mismatch. The result would be a single
spectral peak centered at δk=0. The quasi-phase matching period
Λ may be referred to as the underlying carrier periodicity, and
Kc may be called the carrier wave-vector.

[0077] To obtain multiple quasi-phase matching peaks, a phase modulation
function (PMF) may be applied to the carrier periodicity to achieve a
phase-modulated periodicity. The phase-modulated periodicity, when
applied to the nonlinear optical material in the form of quasi-periodic
poling domains, has the effect of splitting the single spectral peak
centered at δk=0 into multiple spectral peaks, thereby producing
side bands (e.g., phase-matching peaks 162, 164) equally spaced in
wave-vector and adjacent to the center spectral peak. The inverted
domains are referred to as quasi-periodic because the plurality of
domains as a whole is not truly periodic when modulated by the PMF. The
PMF has a very large period compared with the QPM period Λ (i.e.,
the carrier periodicity) and therefore a much smaller k-vector. The PMF
introduces a small perturbation on the relative positions of the poling
domains. As an example, if the side spectral peaks are desired to be 28
cm-1 from the central quasi-phase matching peak, a modulation with a
k-vector of 28 cm-1 may be applied. To apply the phase-modulation
function to the periodic poling, the positions of all inverted domains
along the propagation direction may be shifted by a distance proportional
to the local value of the PMF. The proportionality constant is such that
a phase shift of π dictated by the PMF corresponds to a longitudinal
shift of 0.5Λ for the inverted domain. If the phase-modulating
function is f(x), then the longitudinal shift of the position of the
inverted domain at location x along the propagation direction is
Λf(x)/(2π). It is to be understood that a description of the
same device implementation may be given in terms of frequency (period)
modulation.

[0078] If the pump spectral peaks are symmetrically located around a
center-wavelength λ and spaced in (vacuum) wavelength by
Δλ, then a periodic PMF that can split the phase-matching
peak into components that will allow the generation of up-converted
radiation at wavelengths 0.5(λ-0.5Δλ), 0.5λ, and
0.5(λ+0.5Δλ) should have a period of:

[0079] It is to be understood that the periodic phase-modulation is only
one of a variety of phase-modulating functions that may be utilized for
effective speckle reduction. Though the periodic phase-modulation can
provide near-optimum response in terms of compromise between speckle
reduction and conversion efficiency, other, aperiodic PMF's may also be
used to produce multi-peak up-conversion spectral response, where the
positions and magnitudes of the quasi-phase matching peaks may be
slightly altered compared to the periodic modulation case, while still
providing adequate performance for substantial speckle reduction. It is
the adequate wavelength spacing and the balance of magnitudes of the
quasi-phase matching peaks that may provide optimized speckle reduction
and conversion efficiency.

[0080] In an alternative embodiment, a phase-modulating function can be
applied to the effective refractive index, without interfering with the
periodic poling. For example, by varying the width, thickness, or
refractive index dispersion of the waveguide (in case of waveguide
interaction) δk may be varied periodically, thereby producing
multiple quasi-phase-matching peaks in the spectral response without
altering the truly periodic poling. This technique may also be applied
for phase-matched interactions relying on other means than periodic sign
reversal of the nonlinear coefficient, such as birefringent
phase-matching or intermodal phase-matching (where waveguide dispersion
is used to compensate material dispersion).

[0081] In the case of bulk crystals with birefringent phase-matching or
with QPM, periodic variation of the refractive index by temperature,
stress, or electric field can also be applied for continuous phase
modulation. The modulation of linear optical properties may be produced
by material composition, temperature, electric field, mechanical stress,
or other stress factors. In the case of waveguide propagation, periodic
modulation of a waveguide property such as the waveguide width,
thickness, or material composition, can effect periodic modulation of
phase-matching via the effect on the effective indices of the optical
modes.

[0082] In another embodiment, multiple crystals may be stacked in a
sequence along the direction of light propagation, separated by thin
layers of optically transparent material. The thickness of a separating
layer is selected to provide a phase-difference of (2i+1)π between the
fundamental and the up-converted optical frequencies for discrete
phase-modulation, where i=0, 1, 2, 3 . . . . If the phase-difference is
not an integer number of π, then a more-general PMF with rectangular
phase-modulation may be obtained. Alternatively, blocks of different
media can be stacked together, with different nonlinear and linear
optical properties, leading to modulation of the phase-matching.

[0083] Referring specifically now to FIGS. 8A and 8B, one embodiment of a
phase modulation function and resulting plurality of poling domains
having a phase-modulated periodicity is illustrated. FIG. 8A illustrates
a discrete PMF 150 that consists of a rectangular wave. The discrete PMF
150 effectuates a large-period sign reversal sequence. Every sign
reversal is equivalent to a discrete phase jump of π. For the case of
QPM via periodic poling, this periodic sign reversal is superimposed on
the periodic poling with phase matching period Λ. As illustrated
in FIG. 7, sign reversal of the nonlinear coefficient is equivalent to
dielectric polarization reversal in a ferroelectric crystal such as
lithium niobate. The periodic sign flip is achieved by flipping the
orientation of one or more domains with respect to their nominal
orientation as characterized by the truly periodic poling with phase
matching period Λ.

[0084] The wavelength conversion device illustrated and characterized by
FIGS. 8A-8C is an 8.1 mm-long quasi-periodically poled lithium niobate
waveguide. The discrete PMF 150 has a period of sign-reversal of about
4.5 mm. Phase-matching is not provided outside of the 8.1-mm long region.
Mathematically, the nonlinear optical coefficient outside of the
phase-matched region can be assumed 0 even if the nonlinear medium
extends outside of that region. The duty cycle of the discrete PMF 150
may be altered to achieve the desired spacing and magnitude of the
quasi-phase matching peaks and corresponding two or more
frequency-converted spectral peaks. The duty cycle of the discrete PMF of
the embodiment illustrated in FIG. 8 is approximately 0.5 to achieve the
frequency response with fully suppressed central peak at δk=0.

[0085] As stated above, FIG. 7 illustrates an example of a portion of the
plurality of domains of a wavelength conversion device 130. It is to be
understood that FIG. 7 is not drawn to scale and is for illustrative
purposes only. Referring to FIGS. 7 and 8A, domains having a first
crystallographic orientation 132a (assigned a positive sign) are
indicated by an up-arrow ↑ while domains having a second
crystallographic orientation 132b (e.g., a negative sign) are indicated
by a down-arrow ↓. Referring to FIG. 8A, x=-0.405 on the x-axis
corresponds to the beginning of the phase-matching region of the
wavelength conversion device while x=0.405 is the end of the
phase-matching region of the wavelength conversion device. In the regions
characterized by a positive (+1) sign, the domains alternate periodically
in accordance with the phase matching period Λ. However, at the
transition locations the sign or orientation of the domains is flipped.
As illustrated in FIG. 7, the domains in regions with sign -1 have a
crystallographic orientation flipped compared to the periodic orientation
that would have been present if the strict periodicity defined by the
regions with sign +1 were maintained. The inverted domains in the regions
with sign -1 are phase-shifted by π with respect to normal positions
as defined by the phase matching period Λ (i.e., the carrier
periodicity). The inverted domains in the regions with sign -1 are
shifted longitudinally along x by 0.5Λ (which in this case is
equivalent to flipping the orientation (inverting the sign) of the
domains in the region to be phase-shifted).

[0086]FIG. 8B illustrates the spectral response of a wavelength
conversion device that is poled at a phase-modulated periodicity that is
defined by a phase matching period Λ and modified by multiplying
the periodic domain sign sequence by the PMF depicted in FIG. 8A. The
domain width of each domain is approximately 3.2 μm. The wavelength
conversion device incorporates an MgO-doped congruent lithium niobate
waveguide and quasi-phase matching is used to allow type I phase-matching
using the d33 component of the second-order nonlinear
susceptibility. The spectral response is illustrated in FIG. 8B as a
function of δk. The magnitude of the two main quasi-phase matching
peaks 162, 164 is about 0.455 relative to the magnitude of a single
phase-matching peak produced by a truly periodic QPM device of the same
8.1-mm length. The additional smaller quasi-phase matching peaks do not
contribute to the frequency-converted output. The two main quasi-phase
matching peaks are separated by 27.9 cm-1 in Δk-space. These
quasi-phase matching peaks would quasi-phase match the frequency doubling
of two corresponding infrared wavelengths around 1061 nm that are
separated by about 1 nm in wavelength. Assuming that the laser power P
hops in frequency between the two pump spectral peaks, each
single-longitudinal-mode, the overall efficiency of the second harmonic
generation may be given by:

Pout=0.455η0P2, Eq. (12),

where n0 is the second harmonic generation efficiency of a uniformly
periodically-poled quasi-phase matching grating of the same length with a
single quasi-phase matching peak using a pump laser with a single
longitudinal mode. If the factor of 0.455 renders the conversion
efficiency too low, the efficiency may be recovered by increase of the
peak power in pulsed operation. The external conversion efficiency of the
nonlinear optical device, defined as the ratio of the average power of
the frequency up-converted output signal and the average fundamental
(pump) input power, may be an important parameter describing the
performance of nonlinear device as part of the overall optical system.
The external conversion efficiency is proportional to pump power in the
low-conversion regime and saturates at high conversion. A factor of 2.2
of external efficiency decrease may be compensated by increasing the peak
fundamental power by a factor of 2.2 or more. In addition, an additional
factor of 1.4-2 should be applied when the pulse shape is not
rectangular, but a more typical laser pulse shape such as Gaussian,
hyperbolic secant, or Lorentzian.

[0087] A PMF represented by a periodic sign reversal with period
ΛPM and 50% duty cycle leads to splitting of the QPM spectral
peaks into two components, each having a relative magnitude of
approximately 0.40-0.41 compared to a truly periodic QPM structure of the
same length. In cases where the length of the QPM structure equals only a
few periods of the PMF, deviations of the relative magnitude from
0.40-0.41 can be observed, depending on how the PMF is truncated at the
ends of the QPM structure. Optimum truncation with the highest
utilization of the nonlinear optical response among the two desired QPM
peaks occurs when the PMF is symmetric with respect to the center of the
QPM structure and the length L of the QPM structure equals approximately
(m+0.27)ΛPM, where m is positive integer. For example, when
L=(m+0.3) ΛPM, the relative response magnitude of the two
peaks is about 0.474, 0.442, 0.431, and 0.424 for the cases m=1, 2, 3,
and 4, respectively. Near-optimum truncation with substantially better
than average utilization of the nonlinear response occurs when L is in
the range between about m+0.10 and about m+0.45. Generally favorable
truncation occurs when L is within the range between about m-0.05 and
about m+0.55. Optimum truncation also occurs when the PMF is
anti-symmetric with respect to the center of the QPM structure and the
length L of the QPM structure equals approximately
(m+0.77)ΛPM (or, equivalently, m-0.23)ΛPM, where
m is positive integer. Near-optimum truncation of anti-symmetric PMF
occurs when L is in the range between about m+0.60 and about m+0.95.
Generally favorable truncation of anti-symmetric PMF occurs when L is in
the range between m+0.45 and m+1.05 (or, equivalently, between m-0.45 and
m+0.05). Such is the case depicted in FIG. 8A, where L=1.8
ΛPM. A PMF function with sign flip and 50% duty cycle is
particularly beneficial for generating even numbers of spectral peaks,
especially 2 peaks, because it allows complete suppression of the central
peak at δk=0.

[0088] In another embodiment, the duty cycle of the discrete PMF 150 may
be different from 50%, to limit the suppression of the central peak
located at δk=0. With proper choice of duty cycle, three main
spectral peaks may be obtained with approximately equal magnitude. FIG.
9A illustrates a discrete multiplicative PMF applied to a 7.6-mm device
with the goal of generating three equal QPM peaks, and the spectral
response is illustrated in FIG. 9B (comprising phase-matching peaks 172,
174 and 176). The duty cycle of the PMF is about 0.3. The PMF shows sign
flips which are equivalent to phase jumps equal to π. The QPM peaks
have a magnitude of 0.25 compared to that of a non-modulated QPM device
of the same length. It is noted that the sign, rather than phase, is
provided on the y-axis. The sign imposed by the PMF is compared to that
of a truly periodic structure at the corresponding location.

[0089] In another embodiment, the discrete PMF may control the ratios of
the magnitudes of the multiple phase-matching peaks via the depth of
discrete phase modulation, rather than only by the duty cycle. The PMF
may consists of a rectangular waveform of imparted-phase distribution
with amplitude Φ0PM. The phase imparted on the
phase-matching by the PMF is given by:

where φ0 is a parameter allowing for a constant phase shift of
the PMF waveform with respect to the mid-point along the QPM structure,
if desired. As an example, the PMF for the case L=3.3 ΛPM
with peak ratio of 1:1:1 is illustrated in FIG. 10A, and the spectral
response is illustrated in FIG. 10B (comprising phase-matching peaks 182,
184 and 186). Since the sign function takes on the values +1 when its
argument is positive, -1 when its argument is negative, and 0 when its
argument is 0, the phase defined by the PMF essentially jumps
periodically between the values Φ0PM and
Φ0PM. The depth of phase modulation (DPM) .di-elect cons.
equals twice the amplitude of phase modulation:

.di-elect cons.=2Φ0PM, Eq. (14).

[0090] Using a PMF defined by Eq. (13) with φ0=0 and
ΛPM=0.2335 cm on a wavelength conversion device with
phase-matching length L=0.7706 cm, by using an amplitude
Φ0PM of 0.316π, the three main peaks in the
phase-modulated QPM spectral response have a ratio of 1:1:1. For spectral
response with three near-equal peaks and with near-optimum PMF waveform
truncation, an advantageous range for the phase amplitude
Φ0PM is from about 0.29π to about 0.34π.
Correspondingly, the advantageous range for the depth of modulation is
from about 0.58π to about 0.68π. Similar to the case of a symmetric
PMF enacting periodic sign-flips of the QPM structure described above for
the symmetric discrete (rectangular) PMF defined by Eq. (13), optimum
truncation occurs when the QPM length L is approximately equal to (m+0.3)
ΛPM, where m is again a positive integer. Near-optimum
truncation occurs when the QPM length L is between approximately m+0.15
and m+0.45, and generally favorable truncation occurs when the length L
is between about m-0.05 and about m+0.60. These ranges of near-optimum
and generally favorable truncation apply also for the other types of
periodic symmetric PMFs described below.

[0091] In another alternative embodiment, the PMF is a trapezoidal
function of x. The function is periodic with period ΛPM.
FIGS. 11A and 11B illustrate an example of a trapezoidal PMF for the case
of L=3.3 ΛPM with peak ratio of 1:1:1, and the corresponding
spectral response. In each half-period, rather than taking on a single
constant value, the imparted phase contains a ramp section and a plateau
section. In the case of a symmetric trapezoidal PMF, the latter may be
defined by the phase amplitude ω0PM, the period
ΛPM, the phase shift φ0, and a plateau duty cycle
(PDC). The PDC equals the length of a plateau section as a fraction of
the half-period. The length Lramp of the ramp section is the rest of
the half-period, and the ramp rate is 1/Lramp, multiplied by the
amplitude Φ0PM. Because of the additional free parameter
(ramp), certain required phase-matching peak ratios can be obtained by
different combinations of parameters, and thus different trapezoidal PMF
waveforms. The trapezoidal PMF hence may provide a more flexible method
to modulate the phase-matching to obtain optimum performance in terms of
speckle reduction and conversion efficiency. As an example, a symmetric
trapezoidal PMF (ΛPM=0.2335 cm, φ0=0) with a total
modulation depth of 0.754π (PMF phase amplitude of 0.377π) and PDC
of 0.51 leads to a QPM response with 3 equal peaks of magnitude 0.32
compared to that of a non-modulated QPM device (FIG. 11 B). Efficient QPM
response with three approximately equal peaks is obtained for PMF phase
amplitude in the range between about 0.5π and about 0.60π when the
PDC is 0, and between about 0.29π and about 0.34π when the PDC is
1. Hence, the total range of optimum PMF phase amplitudes is between
about 0.29π and about 0.6π, with most favorable case having a PMF
phase amplitude about 0.377π (total modulation depth of about
0.754π) and PDC of about 0.51. The corresponding range of depths of
modulation is from about 0.58π to about 1.2π.

[0092] The linear ramp of the imparted phase represents a constantly
increasing or decreasing phase shift along the propagation direction. It
can also be seen from a different point of view as a change in the local
period of the QPM structure, in the case of quasi-phase matching. Thus,
the symmetric continuous trapezoidal phase modulation function can be
seen to represent a structure containing three different QPM periods. The
period of the unperturbed design is observed in the regions along the
device length where the PMF is represented by a plateau of the imparted
phase. In the regions where the PMF is represented by ramps, the period
is fixed to one of two values, one smaller, and one larger than the
carrier periodicity, depending on whether the ramp is positive or
negative.

[0093] FIGS. 12A and 12B illustrate another embodiment of phase modulation
that may be utilized to produce at least two frequency-converted spectral
peaks of the wavelength conversion device. The PMF illustrated in FIG. 12
is a sinusoidal function that, when applied to the carrier periodicity
Λ, provides for a plurality of domains having a phase-modulated
periodicity. Rather than discretely modulating the phase of the plurality
of domains as described above with respect to FIGS. 7-10B, the sinusoidal
function has the effect of continuously shifting the position of the
ferroelectric domains with respect to normal periodic positions defined
by the phase matching period Λ. As described above with respect to
the discrete PMFs, the phase-modulated periodicity, when applied to the
nonlinear optical material in the form of quasi-periodic poling, has the
effect of splitting a single quasi-phase matching peak centered at
δk=0 into multiple quasi-phase matching peaks, and producing side
bands equally spaced adjacent to the center quasi-phase matching peak.
The sinusoidal PMF has a very large period compared with the phase
matching period Λ and therefore a very small k-vector. Therefore,
the sinusoidal function introduces a very small perturbation on the
relative positions of the poling domains.

[0094] Referring specifically to FIG. 12A, one embodiment of a PMF and
resulting plurality of poling domains having a phase-modulated
periodicity is illustrated. FIG. 12A illustrates a sinusoidal function
that, when applied to the phase matching period Λ, continuously
shifts the positions of the poling domains by Δx(xl), which
may be expressed by:

where .di-elect cons. is the depth of phase modulation equal to twice the
phase modulation amplitude Φ0PM, and kx is the
k-vector of the sinusoidal PMF. The depth of modulation .di-elect cons.
affects the relative magnitude of the resulting quasi-phase matching
peaks (202, 204, 206, FIG. 12B). For example, the design of FIG. 12A has
a total depth of phase modulation equal to 0.882π.

[0095] As discussed above, in the case of a PMF that is symmetric with
respect to the middle of the QPM length, optimum truncation of the
phase-modulation provided when the relationship between the length of the
phase-modulated QPM structure and the period of the sinusoidal function
is:

L=(m+0.3)*ΛS, Eq. (16),

[0096] where Λs is the period of the sinusoidal function and
m=1, 2, 3 and so on. The range of advantageous values for the ratio
L/Λs is between about m+0.15 and about m+0.45, with an
optimum centered around m+0.3. Generally favorable truncation occurs when
L/Λs is between about m-0.05 and about m+0.6. When the length
of the wavelength conversion device is chosen among these ranges and the
sinusoidal function is applied symmetrically with respect to the center
of the wavelength conversion device, most of the nonlinear spectral
response is preserved among the three desired quasi-phase matching peaks
(e.g., quasi-phase matching peaks 202, 204, 206) and less goes to the
unused quasi-phase matching peaks outside of the central wavelength
region of interest.

[0097] When the length of the wavelength conversion device is chosen in
this manner, the truncation on the two edges of the wavelength conversion
device occurs right after a peak or a trough of the sinusoidal function
and not on or right after a steep slope. The peaks and troughs of the
cosine of the phase are regions where the period of poling has a value
near the center of the range of periods covered by the phase-modulated
QPM-grating. The regions where the phase changes fast on sloped parts of
the sinusoidal function are regions where the local period is
substantially shorter or substantially longer than the average period.
Such periods phase match second harmonic generation for wavelengths that
are away from the central wavelength, thereby spreading the efficiency
outside of the central region of interest. In some embodiments (e.g., 3
wavelengths lasers), it was also determined that for sinusoidal phase
modulation as described by equation 15, the optimum total depth of
modulation c for spectral response with three nearly equal QPM peaks is
between about 0.86π and about 0.90π (phase amplitude
Φ0PM between about 0.43π and 0.45π). In addition, the
spectral response has three dominant approximately equal QPM peaks when
the total depth of phase modulation c is between about 0.8π and about
0.98π (phase amplitude Φ0PM between about 0.4π and
0.49π).

[0098]FIG. 13 schematically illustrates a portion of a wavelength
conversion device 330. The wavelength conversion device 330 comprises a
plurality of poling domains 332 that alternate in crystallographic
orientation as indicated by the up ↑ and down ↓ arrows. The
respective positions of the poling domains 332 are longitudinally shifted
relative to normal periodic positions defined by the carrier periodicity
(depicted by dashed vertical lines 336). The longitudinal shift
illustrated in FIG. 13 is continuous in accordance with the sinusoidal
PMF. However, it should be understood that the longitudinal shift may be
in accordance with PMF other than those that are sinusoidal.

[0099]FIG. 14A illustrates another sinusoidal PMF with period chosen to
allow maximum utilization of the normalized conversion efficiency among 3
main QPM peaks. This may limit the separation of the QPM peaks as the
optimum separation is uniquely associated with the device length. On the
other hand, if the separation is adequate for generating three
independent speckle patterns, and the device length is adequate for
efficient frequency conversion, this design fully utilizes the efficiency
provided by the available interaction length. As illustrated in FIG. 14B,
the response ratio of the three quasi-phase matching peaks 212, 214, 216
is approximately 1:1:1. FIG. 14B illustrates the quasi-phase matching
peaks 212, 214, 216 in δk space while FIG. 14c illustrates the same
three quasi-phase matching peaks 212', 214', 216' in wavelength space for
a fabricated 4.8-mm long quasi-periodically-poled MgO-doped lithium
niobate waveguide. The experimental ratio of the quasi-phase matching
peak magnitudes may vary somewhat compared with the theoretical ratio due
to imperfect device fabrication. Such imperfections may include, but are
not limited to, low-to-moderate poling fidelity and some waveguide
non-uniformity along the length in the case of wavelength conversion
devices based on waveguides. Small variations of the quasi-phase matching
peak ratio will typically have a small effect on the speckle reduction
properties.

[0100] As noted above, the expected perceived speckle contrast reduction
(by time averaging in the human eye) is {square root over (2)} if the
laser alternates between two narrow spectral lines (i.e., fundamental
spectral peaks) with sufficient separation and {square root over (3)} if
it alternates between three lines for consecutive projected image frames.
Speckle reduction may be further increased from those values if each of
the lines emitted by the laser is spectrally broadened. FIG. 15
illustrates measured speckle contrast versus spectral width for copy
paper as a projection screen material. As shown in FIG. 15, even for a
spectral width of only 0.3 nm additional 20% reduction of speckle
contrast is measured when white copy paper serves as the projection
screen material. Further, a spectral width of 0.1 nm for the frequency
converted spectral peaks (full width at half maximum in the green
spectral range) may reduce speckle contrast by about 10% (which is about
0.2 nm in the infrared spectral range). Therefore, some embodiments may
utilize a spectral width of greater than 0.1 nm in the
frequency-converted range to provide for some additional speckle contrast
reduction. Since, as also noted above, it is advantageous for the
practice of the embodiments described herein to operate the pump laser in
pulsed mode, to compensate for conversion efficiency decrease introduced
by aperiodic poling, some spectral broadening of each emitted spectral
peak may be relatively easy to achieve. Various techniques for pulsing
semiconductor lasers (gain-switching, Q-switching, and hybrid
gain/Q-switching) may be utilized.

[0101] In pulsed operation, the spectral shape and width of the diode
optical output closely follows the shape and width of the spectral
response of its cavity reflectors (the DBR mirror in case of DBR lasers).
Therefore, to broaden the emitted spectral peak, the spectral bandwidth
of the DBR mirror may be increased, e.g., by chirping the period of the
built-in diffraction grating and/or increasing the coupling parameter
(grating "strength"). When the DBR laser is switched (fast tuned) to a
different central wavelength (by changing temperature and/or bias current
of the DBR section), the spectral width of the emitted spectral peak will
remain approximately the same. Therefore, the speckle contrast reduction
due to line broadening (e.g., as predicted by the graph of FIG. 15) will
be added to the reduction produced by time averaging of the wavelength
hopping. If the spectral peak or line is broadened to about 0.4 nm and
the central wavelength is switched between three positions separated by
about 0.4 nm or more, then the expected perceived speckle contrast
reduction for the white copy paper projector screen material may be as
much as {square root over (2)} {square root over (3)}= {square root over
(6)}.

[0102] An additional advantage of broadening the frequency-converted
spectral peak(s) of the green laser optical system is that it makes the
laser optical system compatible with a passive polarization scrambling
technique. Passive polarization split-and-delay units convert small
variations of the laser output wavelength into large variations of the
polarization state. When such a device is inserted in the optical path of
the beam emitted by the laser, an additional {square root over (2)}
speckle contrast reduction may result. If the fundamental spectral
peaks(s) are broadened to 0.4 nm, the size of the split-and-delay unit
could be very small, on the order of 1 mm Switching between three central
wavelength positions, the total reduction of the speckle contrast will be
up to {square root over (2)}* {square root over (2)}* {square root over
(3)}= {square root over (12)}.

[0103] Regarding spectral line broadening, the high-power DBR lasers
typically have a relatively long cavity (2-4 mm) and non-zero reflection
at the front facet, which means that their round-trip cavity loss
spectrum is a superposition of a DBR mirror reflection and periodic
Fabry-Perot peaks. When the DBR mirror reflection bandwidth is increased,
several Fabry-Perot peaks will fit inside it, resulting in the laser
emitting not a one line, but several closely spaced lines (Fabry-Perot
modes). This will change the conversion efficiency for second harmonic
generation and should be taken into consideration when designing
wavelength-switched lasers according to the present disclosure. Generally
speaking, if each fundamental spectral peak contains N modes, an increase
in efficiency close to (2-1/N) will be realized compared to the
single-mode fundamental peak case, slightly less because the N modes will
not all have the same intensity (power).

[0104] While the broader the spectral line the better the speckle
reduction, for preserving conversion efficiency the overall spectral
bandwidth of each of the pump laser spectral peaks should not be
significantly larger than the QPM bandwidth of each of the quasi-phase
matching peaks. For example, if the pump laser fundamental spectral peak
has a spectral width of 0.3 nm in wavelength space, then a quasi-phase
matching FWHM bandwidth of any of the two or three quasi-phase matching
peaks that is significantly smaller than 0.3 nm may substantially
decrease the conversion efficiency provided by that QPM peak. Such a
decrease in conversion efficiency may also depend on whether the spectral
width of the fundamental spectral peaks is dominated by frequency chirp
(variation of the central optical frequency of the pulse over the
duration of the pulse) or by the simultaneous generation of many
longitudinal modes. If the spectral width of the fundamental spectral
peaks is dominated by frequency chirp, the reduction in efficiency due to
QPM spectral filtering may be more significant. The efficiency reduction
due to limited QPM-peak bandwidth may impose limits on the length of the
wavelength conversion device or on the complexity of the PMF used to
produce the multiple quasi-phase matching peaks.

[0105] As an example and not a limitation, a three-section 1060 nm DBR
laser was used to pump a SHG wavelength conversion device 130 which was
poled to QPM at three pump wavelengths: 1059.9 nm, 1060.5 nm, and 1061.1
nm (FIG. 14c). FIG. 1 shows the experimental setup. The fast wavelength
switching of the pump laser was realized by applying a 65 Hz electrical
square wave to the DBR heater of the DBR section electrode 102 to
thermally change Bragg-resonance wavelength of the DBR section 112. The
pump laser was gain-switched by applying a sinusoidal signal superimposed
on a DC current to the gain section. By properly setting the heat sink
temperature of the pump laser 110 and the amplitude of the electrical
square wave signal, the output wavelength of the pump laser was fast
switched between two of the three fundamental spectral peak wavelengths.
Thus, the output from the wavelength conversion device 130 can be fast
switched between any two of the three frequency-converted spectral peaks
having approximate green wavelengths λ1=530 nm,
λ2=530.3 nm and λ3=530.6 nm FIGS. 16A-16C depict
the output spectra (average of 20 measurements) of the
wavelength-switched green laser for three different operation modes. The
switching time between λ1 and λ3 is about 1.5 ms
(FIG. 16c), and the switching time between λ1 and
λ2 (FIG. 16A) or λ2 and λ3 (FIG. 16B) is
about 1.0 ms. With an appropriate electrical waveform with plateaus at
three different levels applied to the DBR heater, optical output at all
three different green wavelengths may be produced within the integration
time of the eye, allowing for {square root over (3)} speckle-contrast
reduction.

[0106] As another example and not a limitation, a three-section DBR laser
was used to pump a SHG waveguide which was poled to QPM at room
temperature mainly at two pump wavelengths: 1059.7 nm and 1060.8 nm. The
tuning curve of the SHG waveguide is shown in FIG. 17. The exemplary SHG
waveguide QPM design is based on sinusoidal phase modulation with large
depth of modulation to produce two main side peaks and reduced central
peak. Side peaks with normalized efficiency greater than about 1/3 of the
efficiency of a truly periodically poled device of the same length are
produced when the depth of sinusoidal phase modulation is between about
0.92π and about 1.4π, with optimum range between and about
1.04π1.3π. The wavelength of the pump laser was alternated between
the two phase-matched wavelengths by applying a 65 Hz electrical square
wave to the DBR heater. The pump laser was gain-switched by applying a
sinusoidal signal superimposed on a DC current to the gain section. FIG.
2 illustrates the output spectrum (average of 22 measurements) of the
green laser with laser wavelength alternating between
λ1=529.85 nm and λ2=530.40 nm. To measure the
speckle contrast, the non-collimated green output beam was projected on
regular printer white paper which was taped on a non glossy back board.
FIG. 18A illustrates the measured image brightness histogram of the
output of the wavelength-alternated green laser. For comparison, the
measured histogram of a CW single-wavelength green output is shown in
FIG. 18B. The speckle contrast was reduced from 63.2% to 43.1% by
alternating the laser wavelength, which corresponds to a reduction of
slightly more than {square root over (2)}.

[0107] It is noted that due to the saturation of conversion efficiency at
high peak power, using phase-modulated QPM devices in the pulsed regime
may come at virtually no cost in terms of conversion efficiency in
comparison with the same pulsed regime with uniform QPM structure of the
same length. FIG. 19 shows the results of a simulation of second harmonic
output power as a function of input power when the peak power is high
which normally leads to saturation of SHG conversion efficiency when the
QPM structure is strictly periodic. The x-axis is infrared input power,
while the left y-axis is green output power in watts and the right y-axis
is normalized efficiency. Plot 504 is the efficiency of a QPM structure
with constant carrier periodicity and periodic sign flips (phase shifts
of π) while plot 506 is a the efficiency of a uniformly poled crystal.
Plot 502 shows that at a peak power of 2 watts the conversion efficiency
with the sign-modulated QPM structure is over 90% of that with the
uniformly poled crystal. The lower normalized conversion efficiency of
the aperiodic QPM structure leads to offset of the saturation of
conversion efficiency to higher pump-power levels. This substantially
reduces the gap in conversion efficiency between the strictly periodic
QPM device and the phase-modulated QPM device at high peak optical power.
The saturation of SHG conversion efficiency at high optical power leads
to a reduction of the area of the normalized-efficiency tuning curve
showing the conversion efficiency in %/W as a function of wavelength.
Spreading the phase-matching over a larger wavelength range by
phase-modulation of the QPM structure allows to recover some or most of
that loss of efficiency-bandwidth area and utilize it for speckle
reduction,

[0108] It is to be understood that the preceding detailed description is
intended to provide an overview or framework for understanding the nature
and character of the subject matter as it is claimed. It will be apparent
to those skilled in the art that various modifications and variations can
be made to the embodiments described herein without departing from the
spirit and scope of the invention. Thus, it is intended that the present
invention cover the modifications and variations of this invention
provided they come within the scope of the appended claims and their
equivalents.

[0109] It is noted that terms like "preferably," and "typically," when
utilized herein, are not intended to limit the scope of the claimed
invention or to imply that certain features are critical, essential, or
even important to the structure or function of the claimed invention.
Rather, these terms are merely intended to highlight alternative or
additional features that may or may not be utilized in a particular
embodiment of the present invention. Further, it is noted that reference
to a value, parameter, or variable being a "function of" another value,
parameter, or variable should not be taken to mean that the value,
parameter, or variable is a function of one and only one value,
parameter, or variable.

[0110] For the purposes of describing and defining the present invention
it is noted that the terms "substantially," "approximately" and "about"
are utilized herein to represent the inherent degree of uncertainty that
may be attributed to any quantitative comparison, value, measurement, or
other representation. The term "substantially" is also utilized herein to
represent the degree by which a quantitative representation. e.g.,
"substantially above zero," varies from a stated reference, e.g., "zero,"
and should be interpreted to require that the quantitative representation
varies from the stated reference by a readily discernable amount.